In recent years, realization of high recording density and/or large recording capacity of optical recording media represented by optical disc is being advanced. For example, there has been put to practical use “DVD (Digital Versatile Disc: a Trade Name)” (hereinafter referred to as DVD)” in which disc having the same diameter as that of “CD (Compact Disc: Trade Name (hereinafter referred to as CD)” where, e.g., object lens (objective) numerical aperture (NA) of the optical pick-up unit is 0.45, wavelength of a beam (laser beam) for signal read-out is 780 nm, disc transmission base thickness (which refers to thickness of light transmission layer provided on recording layer of optical disc) is 1.2 mm and recording capacity is about 650 MB is used, object lens (objective) numerical aperture (NA) of optical pick-up unit is caused to be 0.60, wavelength of a beam (laser beam) for signal read-out is caused to be 650 nm, disc transmission base thickness is caused to be 0.6 mm, and recording capacity is enhanced to 4.7 GB which is about seven times greater than that of CD.
Further, in this DVD, in order to approximately double recording capacity, two-layer recording in which two layers are provided at spacing of several ten μm is also realized.
As a technology which serves as key in realization of high recording density and large recording capacity as stated above, there are “high NA” of object lens (objective) and “multi-layer recording” in the optical recording medium.
However, in realization of “high NA” or “multi-layer recording”, there take place problems as described below.
First, according as numerical aperture (NA) of the object lens (objective) becomes greater, spherical aberration produced by deviation quantity Δt from reference value of disc transmission base thickness increases in proportion to fourth power of numerical aperture (NA). Namely, quantity produced of spherical aberration is indicated as below.[Spherical aberration]∝Δt {(n2−1)/n3}NA4/λ
In the above formula, n is refractive index of disc base, and λ is wavelength of a beam (laser beam) for signal read-out. Namely, as understood from the above formula, according as “high NA” is realized, error quantity tolerated with respect to the disc transmission base thickness is remarkably decreased.
Here, when ratios of deviation quantities Δt from the reference value tolerated with respect to the disc transmission base thickness when quantity produced of spherical aberration is assumed to be constant in connection with three cases of the above-described CD and DVD, and comparative example in which further high density is assumed where “numerical aperture (NA)=0.85 and a beam wavelength=405 nm” are calculated, there are obtained results as described below.
When Δt at CD (NA=0.45, λ=780 nm) is assumed to be 1,
Δt at DVD (NA=0.60, λ=650 nm) is equal to 0.264, and
Δt at the comparative example (NA=0.85, λ=405 nm) is equal to 0.0409.
Tolerable deviation quantity Δt from the reference value is 0.264 times at DVD with respect to CD, and is 0.155 times at the comparative example with respect to DVD. Namely, it is understood that tolerable deviation quantity Δt from the reference value is decreased to so far as about 1/25 as compared to CD in the condition of the comparative example.
In addition, in the “multi-layer recording” as effective system of realization of high recording capacity, plural layers different in the disc transmission base thickness are intentionally provided in a stacked manner. For this reason, quantities produced of spherical aberration at the convergent point become different values every respective layers.
When attempt is made to carry out “high NA” or “multi-layer recording”, etc. in order to realize high recording density and large recording capacity as stated above, characteristic degradation based on increase in spherical aberration produced resulting from error of the disc transmission base thickness becomes problem also in both cases.
On the contrary, e.g., as disclosed in the Japanese Patent Application Laid Open No. 269611/1998 publication, there is proposed a technique for forming spherical aberration correction pattern by using liquid crystal panel, as shown in FIG. 1, in order that there result optimum aberrations every respective layers in carrying out “multi-layer recording” to carry out aberration correction.
In this FIG. 1, the abscissa indicates radial position in the case where normalization is made so that radius in the correcting element for a beam converged onto the recording layer of the optical recording medium becomes equal to 1, and the ordinate indicates phase change quantity given to a beam by the aberration correcting element.
Moreover, FIG. 2 shows phase change quantity given to a beam for signal read-out by the aberration correcting element in a manner classified into phase change quantity for correction of spherical aberration and phase change quantity for defocus correction, wherein the abscissa indicates radial position in the case where normalization is made so that radius at the correcting element for the beam converged onto the recording layer of the optical recording medium becomes equal to 1, and the ordinate indicates phase change quantity given to the beam by the aberration correcting element.
There exists the relationship that when difference between two phase change quantities shown in this FIG. 2 is taken, pattern of phase change quantity as shown in FIG. 1 is obtained When attempt is made to give only phase change quantity for correction of spherical aberration to the beam by the correcting element without including phase change quantity for defocus correction, its phase difference becomes large. For this reason, phase change quantity is given in the state including phase change quantity for defocus correction.
Namely, the phase distribution shown in FIG. 1 corresponds to the distribution obtained by taking difference between phase distribution (−r4) corresponding to spherical aberration and defocus pattern (−r2) separately shown in FIG. 2, and is frequently used in carrying out aberration analysis, etc. in an ordinary sense.
FIG. 3A is an explanatory view showing the procedure in the case where both focus bias value and spherical aberration correction quantity that the aberration correcting element gives are optimized by using the phase distribution shown in FIG. 1. Here, FIG. 3A represents signal characteristic by contour line, wherein the ordinate indicates phase correction quantity that the aberration correcting element gives, and the abscissa indicates focus bias value. In addition, FIG. 3B is an explanatory view showing change of spherical aberration quantity and defocus quantity adjusted by the phase correction quantity and the focus bias value shown in FIG. 3A by using the coordinate axis intelligibly modified, wherein the ordinate indicates spherical aberration quantity and the abscissa indicates defocus quantity.
Here, in the case where liquid crystal panel is used as an aberration correcting element 4 and there is employed a configuration to correct spherical aberration by aberration correction pattern as shown in FIG. 1 as disclosed in the Japanese Patent Application Laid Open No. 269611/1998 publication, problems as described below take place.
It is to be noted that phase distribution close to the pattern shown in FIG. 1 is caused to be generated in a pseudo manner by step-shaped pattern based on division of electrode pattern in the Japanese Patent Application Laid Open No. 269611/1998 publication. On the other hand, even if such step-shaped pattern is not employed, technology for generating continuous phase distribution is announced in, e.g., “4p-K-1 of proceedings of academic lecture meeting of autumn society of applied physics, 2000” or “CPM 2000-91 (2000-09) “Technical Research Report of Institute of Electronics and Communication Engineers of Japan” Society of Electronic Information and Communication”, etc. In this technology, electrodes positioned at the inner circumferential side and the outer circumferential side of the liquid crystal panel are used to generate electric field in a direction along the principal surface in place of thickness direction of the panel to form potential gradient in the direction of the panel surface within the liquid crystal layer. However, even if such a liquid crystal panel which generates continuous phase distribution is used as the aberration correcting element, problem as described below similarly takes place.
Namely, in the optical recording medium using object lens (objective) of “high NA” or “multi-layer recording”, etc., in the case where the signal characteristic is optimized, it is necessary to optimize both focus bias value and spherical aberration correction quantity by the liquid crystal.
However, in the case where attempt is made to carry out such optimization by using the aberration correction pattern of FIG. 1, it can be confirmed that the signal characteristic when focus bias value and correction quantity by liquid crystal are changed results in contour line distribution as shown in FIG. 3A.
Accordingly, in the case where adjustment is carried out from the “initial position” toward the “best position” in the figure, if “setting of focus bias” and “setting of liquid crystal correction quantity” are not alternatively repeated many times, it is impossible to follow up so that there results the “best position”.
This not only allows the adjustment to be complicated, but also leads to the fact that adjustment is converged into the point which is not the “best position” by a little factor.
This can be considered in a manner as described below.
For the purpose of simplifying the explanation, the signal characteristic with respect to defocus quantity and spherical aberration is assumed to be a characteristic as shown in FIG. 3B. Here, as the signal characteristic, amplitude of RF signal or jitter of RF signal, etc. may be used.
As described above, by first changing “focus bias”, it is possible to adjust “defocus quantity” without affecting “spherical aberration”.
However, “spherical aberration” can be corrected by “phase control by liquid crystal”, and, on the other hand, gives change also to formation of light spot on a light detecting element for forming focus error signal and intensity distribution by “phase change by liquid crystal” produced when spherical aberration quantity is controlled. For this reason, followed by “phase control by liquid crystal”, “focus bias” where signal characteristic becomes satisfactory would change (In practice, the best image surface position where the signal characteristic becomes best also somewhat changes by “spherical aberration control”.
Accordingly, in the case where the “focus bias” is caused to be constant, and the “spherical aberration quantity” is caused to be changed, defocus quantity also changes followed by change of “spherical aberration quantity”. However, phase distribution corresponding to essential aberration required in correcting spherical aberration produced by error of the disc transparent base thickness is only the phase distribution (−r4) corresponding to the spherical aberration.
Further, change of aberration quantity in the case where the phase distribution shown in FIG. 1 is used to carry out aberration correction can be expressed in a manner described below by using variable C.[Pattern 0]=C{(−r4)−(−r2)}  (1)
Conventional adjustment can be expressed as the case where it is assumed that this variable C is changed.
Namely, in the conventional correction, the phase distribution shown in FIG. 1 is collectively changed in the ordinate direction by variable C.
As stated above, in the case where correction is made by using aberration correction pattern disclosed in this Japanese Patent Application Laid Open No. 269611/1998, there was the problem that adjustment of correction quantity becomes complicated.